Spectroscopic observation and geometry assignment of the minimum energy conformations of methoxy-substituted benzenes

Breen, Philip J.; Bernstein, E. R.; Secor, Henry V.; Seeman, Jeffrey I.

doi:10.1021/ja00188a002

Cited by 96 publications

(74 citation statements)

References 5 publications

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“…60 This conformation, with the C X -C Y bond perpendicular to the ring, both minimizes steric repulsion and allows hyperconjugation between the C X -C Y bond of the chain and aromatic system. 71 This structure is stabilized by conjugation between the lone pair electrons in the oxygen p-type orbital and the aromatic system that dominates over repulsive steric forces. In addition, a feature common to alkyl-amine chains is a preference for gauche conformers which are stabilized through a NH¯ interaction with the ring.…”

Section: The Conformational Preferences Of Related Single-chain mentioning

confidence: 99%

Entropy-driven population distributions in a prototypical molecule with two flexible side chains: O-(2-acetamidoethyl)-N-acetyltyramine

Shubert

Baquero

Clarkson

et al. 2007

The Journal of Chemical Physics

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Resonant two-photon ionization (R2PI), resonant ion-dip infrared (RIDIR), and UV-UV hole-burning spectroscopies have been employed to obtain conformation-specific infrared and ultraviolet spectra under supersonic expansion conditions for O-(2-acetamidoethyl)-N-acetyltyramine (OANAT), a doubly substituted aromatic in which amide-containing alkyl and alkoxy side chains are located in para positions on a phenyl ring. For comparison, three single-chain analogs were also studied: (i) N-phenethyl-acetamide (NPEA), (ii) N-(p-methoxyphenethyl-acetamide) (NMPEA), and (iii) N-(2-phenoxyethyl)-acetamide (NPOEA). Six conformations of OANAT have been resolved, with S(0)-S(1) origins ranging from 34,536 to 35,711 cm(-1), denoted A-F, respectively. RIDIR spectra show that conformers A-C each possess an intense, broadened amide NH stretch fundamental shifted below 3400 cm(-1), indicative of the presence of an interchain H bond, while conformers D-F have both amide NH stretch fundamentals in the 3480-3495 cm(-1) region, consistent with independent-chain structures with two free NH groups. NPEA has a single conformer with S(0)-S(1) origin at 37,618 cm(-1). NMPEA has three conformers, two that dominate the R2P1 spectrum, with origin transitions between 35,580 and 35,632 cm(-1). Four conformations, one dominate and three minor, of NPOEA have been resolved with origins between 35,654 and 36,423 cm(-1). To aid the making of conformational assignments, the geometries of low-lying structures of all four molecules have been optimized and the associated harmonic vibrational frequencies calculated using density functional theory (DFT) and RIMP2 methods. The S(0)-S(1) adiabatic excitation energies have been calculated using the RICC2 method and vertical excitation energies using single-point time-dependent DFT. The sensitivity of the S(0)-S(1) energy separation in OANAT and NPOEA primarily arises from different orientations of the chain attached to the phenoxy group. Using the results of the single-chain analogs, tentative assignments have been made for the observed conformers of OANAT. The RIMP2 calculations predict that interchain H-bonded conformers of OANAT are 25-30 kJ/mol more stable than the extended-chain structures. However, the free energies of the interchain H-bonded and extended structures calculated at the preexpansion temperature (450 K) differ by less than 10 kJ/mol, and the number of extended structures far outweighs the number of H-bonded conformers. This entropy-driven effect explains the presence of the independent-chain conformers in the expansion, and cautions future studies that rely solely on relative energies of conformers in considering possible assignments.

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Section: The Conformational Preferences Of Related Single-chain mentioning

confidence: 99%

Entropy-driven population distributions in a prototypical molecule with two flexible side chains: O-(2-acetamidoethyl)-N-acetyltyramine

Shubert

Baquero

Clarkson

et al. 2007

The Journal of Chemical Physics

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“…This could be due to the orientation of the OCH 3 group which is, as reported for related molecules and will be further discussed, forced to be out of the plane of the phenyl ring by the neighboring benzylic groups. In the absence of steric hindrance, the planar arrangement has been clearly established by X-ray [22,23], optical spectroscopic [24], TOFMS [25] and computational [26] methods, and this arrangement is necessary for the OCH 3 to have its full activating effect. Such is not the case for the OH group which is not only a better donor than an OCH 3 group [27] but also made potentially more activating by the presence of a ring oxygen-to-hydrogen hydrogen bond [18].…”

Section: May-jun 2006mentioning

confidence: 99%

Synthesis, properties and X‐ray structure of 5‐azido‐2‐methoxy‐1,3‐xylyl‐18‐crown‐5

Yonekawa¹,

Goodpaster²,

Abel³

et al. 2006

Journal of Heterocyclic Chem

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5-Azido-2-methoxy-1,3-xylyl-18-crown-5 has been prepared by reacting p-toluenesulfonyl azide with the carbanion generated from the reaction of 5-bromo-2-methoxy-1,3-xylyl-18-crown-5 with n-butyl lithium. The asymmetric N 3 stretch of this product has been observed as a single band at 2110 cm -1 in dichloromethane solution. Addition of solid NaSCN, KSCN and CsSCN shifts this band to 2115, 2113 and 2112 cm -1 , respectively. Computational studies of this azide at the B3LYP-6-31G* level in the presence and absence of Na + predicted these bands to be at 2173 cm -1 and 2184 cm -1. For the salt-containing solutions, additional bands were observed at 2066 cm -1 , 2056 cm -1 and 2055 cm -1 , respectively, which are in the range expected for CN stretches. The X-ray structure of this azide has been determined. The terminal and internal N-N bond lengths were found to be 1.127(2) and 1.245(2) , respectively, which is the usual pattern for aromatic azides. The crown ether is looped over the face of the aromatic ring resulting in an angle of 38.94° between the plane defined by the aromatic ring and that defined by the five ring oxygen atoms. In addition, the CH 3 group is rotated out of the plane of the phenyl ring with C1-C18-O181-C182 and C17-C18-O181-C182 dihedral angles of 93.81(14)° and -90.54(14)°, respectively.

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“…They could show that all bands in the vibronic spectrum have the same rotational constants in the ground state and belong therefore to the same rotamer, which was identified from a comparison to quantum chemical calculations as the trans ‐1,2‐DMB rotamer. Three bands, labeled A , B , and C at 36101.5, 36163.9, and 36256.9 cm −1 in the R2PI molecular beam spectrum of 1,3‐dimethoxybenzene (1,3‐DMB) have been assigned to the origins of three different rotamers by Breen et al . Yang et al .…”

Section: Introductionmentioning

confidence: 99%

Excited‐State Dipole Moments and Transition Dipole Orientations of Rotamers of 1,2‐, 1,3‐, and 1,4‐Dimethoxybenzene

et al. 2018

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Rotationally resolved electronic Stark spectra of rotamers of 1,2-, 1,3-, and 1,4-dimethoxybenzene have been recorded and analyzed using evolutionary strategies. The experimentally determined dipole moments as well as the transition dipole moments are compared to the results of ab initio calculations. For the electronic ground states of the experimentally observed dimethoxybenzenes, the permanent dipole moments can be obtained from vectorial addition of the monomethoxybenzene dipole moment. However, this is not the case for the electronically excited states. This behavior can be traced back to a state mixing of the lowest electronically excited singlet states for the asymmetric rotamers. For the symmetric rotamers however, this is not valid. We discuss several possible reasons for the non-additivity of the dipole moments in the excited states of the symmetric rotamers.

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Spectroscopic observation and geometry assignment of the minimum energy conformations of methoxy-substituted benzenes

Cited by 96 publications

References 5 publications

Entropy-driven population distributions in a prototypical molecule with two flexible side chains: O-(2-acetamidoethyl)-N-acetyltyramine

Entropy-driven population distributions in a prototypical molecule with two flexible side chains: O-(2-acetamidoethyl)-N-acetyltyramine

Synthesis, properties and X‐ray structure of 5‐azido‐2‐methoxy‐1,3‐xylyl‐18‐crown‐5

Excited‐State Dipole Moments and Transition Dipole Orientations of Rotamers of 1,2‐, 1,3‐, and 1,4‐Dimethoxybenzene

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